Pattern reading apparatus

ABSTRACT

A pattern reading apparatus includes an objective lens positioned opposite an object surface, the object surface being a reflection surface having a pattern formed thereon as an object to be read. A minute-area light source is positioned to be conjugate with a center of curvature of the object surface through the objective lens, for illuminating the object surface through the objective lens. An imaging lens is positioned farther from the object surface than the light source, with the optical axis of the imaging lens being coincident with the objective lens. An imaging element reads the image of the pattern which is reflected at the object surface and formed through the objective lens and the imaging lens.

[0001] This is a division of U.S. patent application Ser. No.08/916,408, filed Aug. 22, 1997, the contents of which are expresslyincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a pattern reading apparatus forreading a pattern formed on a surface of a silicon wafer or the like,and more specifically, to a pattern reading apparatus for reading apattern formed on a reflective or transparent surface.

[0003] In manufacturing semiconductor products, semiconductor layers areapplied to a semiconductor substrate, such as a silicon wafer or thelike, by vapor deposition and then design patterns are formed byphoto-lithography processes, etching processes, and the like. Ingeneral, a serial number is applied to the silicon wafer by laseretching so that the silicon wafer can be tracked during the patternforming processes based on the serial number. Conventionally, the serialnumber on the silicon wafer is discriminated by a worker visuallyexamining the wafer.

[0004] However, since the silicon wafer is mirror finished, for a workerto read the serial number, the wafer must be viewed obliquely whileholding it to the light, or by some similar method. Further, since thequality of the pattern may deteriorate as the silicon wafer is subjectedto processes such as etching, vapor deposition and the like, it isparticularly difficult to discriminate the serial number of the siliconwafer after a number of such processes.

[0005] Conventionally, two types of pattern reading devices have beenknown: a reflective-type reading device, and a transmission-type readingdevice. The former is used for reading a pattern formed on a reflectivesurface, and the latter is used for reading a pattern formed on atransmission-type surface.

[0006] In an example of the reflective-type reading device, lightemitted by a light source is incident, through a lens, to a surface onwhich the pattern is formed, and an image of the pattern is formed by animaging lens on a screen or the like. In this case, a portion of thelight incident to the lens is reflected on a surface of the lens tocreate ghosting light, which reaches the screen and reduces contrast ofthe image of the pattern. Further, the specular reflection from thesurface having the pattern formed thereon may be incident on the screenmaking it more difficult to observe the image of the pattern.

[0007] As an example of the transmission-type reading device, a knowndevice has a Fourier transformation lens, that is used for reading apattern formed on a light-transmission-type object by subjecting thepattern to a predetermined filter processing. In these optical systems,the light beam from a point light source passes through a first lens andis incident on an object as a parallel light beam. After passing throughthe object, the light beam is converged by a second lens and caused topass through a spatial filter disposed at the back focal point of thesecond lens. When an imaging lens, having the front focal point set tothe position of the filter, is disposed behind the filter, an objectimage, which is affected by the function of the filter, is formed at theback focal point of the imaging lens.

[0008] For example, to output an emphasized image of a pattern formed onan object surface, a high-pass filter may be used as the spatial filterto shade the paraxial rays which correspond to the image of the pointlight source. Further, an imaging element may be disposed at the imagingposition to capture and process the image for further processing ordisplaying on a display unit.

[0009] In the above conventional filtering optical system, however, whenan objective lens (first lens) has spherical aberration such as, forexample, a spherical single lens or when coma and curvature of fieldarise because a light beam is obliquely incident on the objective lens,there is a problem in that the light beam which forms the image of apoint light source does not converge to a point but scatters over alarger area such that a large shading region must be provided toproperly execute filtering. Thus, a quantity of light used to form theimage is lowered.

[0010] In a pattern reading apparatus using the above conventionalfiltering optical system, since the magnification of a pattern imagehaving been formed cannot be changed, the pattern image cannot beoptically enlarged or reduced. That is, since an object surface isdisposed to the focal point of an objective lens in the conventionaloptical system, the light beam emitted from the objective lens is madeafocal. Thus, even if the imaging lens is moved, magnification cannot bechanged. To change the magnification, the imaging lens must be composedof a group of a plurality of lenses.

[0011] Further, a pattern reading apparatus using the above conventionalfiltering optical system cannot be easily used when the object to beread is intended to function as a prism (i.e., has a wedge shape or thelike) for deflecting a light beam. In this case, the image of the pointlight source will not be shaded by a spatial filter because the imagewill be formed at a position outside of the axis. Thus, a component oflight other than the scattered reflected component will be incident onan imaging lens and a desired filtered output image cannot be output. Asimilar problem also may arise when a reflection surface is tilted atthe time a pattern is read by this type of apparatus.

SUMMARY OF THE INVENTION

[0012] A first object of the present invention is to provide a patternreading apparatus capable of forming a high-contrast image of anindistinct pattern such as a serial number or the like formed on amirror surface such as a silicon wafer, and in particular, capable ofeven reading a pattern which has deteriorated because of processing suchas etching, vapor evaporation, and the like.

[0013] A second object of the present invention is to provide a patternreading apparatus capable of reading a pattern image even if a portionof an illumination light beam is reflected at the lens surface of anobjective lens or even if an object surface is somewhat irregular.

[0014] A third object of the present invention is to provide a patternreading apparatus, which includes a filtering optical system, capable ofshading the light beam that forms the image of a point light sourcewithout lowering the quantity of light of the pattern imagesubstantially, even if the image of the point light source is expandeddue to spherical aberration, coma, and curvature of field of anobjective lens.

[0015] A fourth object of the present invention is to provide a patternreading apparatus using a filtering optical system in which themagnification of a pattern image may be changed using a simplestructure.

[0016] A fifth object of the present invention is to provide a patternreading apparatus, using a filtering optical system, with which an imageof a point light source and a shading region of a spatial filter can bemade to coincide, even if an object has a function of a prism or even ifa reflection type object has a tilted reflection surface.

[0017] According to an aspect of the present invention, there isprovided, a pattern reading apparatus including a minute-area lightsource, an objective lens, an imaging lens, and an imaging element. Theobjective lens causes the illumination light beam from the light sourceto be incident on a reflection surface having a pattern formed thereonas an object to be read and converges the light beam reflected from thereflection surface. The imaging lens is for imaging an image of thepattern by a scattered reflected component, which has passed through theobjective lens, of the reflected light beam. The imaging element isdisposed at a position where the image of the pattern is imaged forreading the pattern. The light source is optically conjugate with acenter of curvature of the surface of the object to be read through theobjective lens.

[0018] According to another aspect of the present invention, there isprovided, a pattern reading apparatus including illumination means forilluminating a reflection surface having a pattern formed thereon as anobject to be read by a parallel light beam and detection means fordetecting an image by imaging a scattered reflected component ofillumination reflected from the reflection surface.

[0019] According to yet another aspect of the present invention, thereis provided, a pattern reading apparatus, an objective lens, a spatialfilter, and an imaging lens. The imaging lens forms the image of thepattern using the light beam that passes through the spatial filter. Thepattern reading apparatus includes a minute-area light source forcausing an illumination light beam to be incident on an object surfacehaving a pattern formed thereon as an object to be read. The objectivelens converges a light beam carrying the information of the pattern. Thespatial filter is disposed at a position where a size of an image of thelight source formed by the objective lens is smaller than a size of theimage at a paraxial image point. The spatial filter has a shading regionfor shading a portion of the light beam that forms an image of the lightsource from the light beam.

[0020] According to yet another aspect of the present invention, thereis provided, a pattern reading apparatus including a minute-area lightsource, an objective lens for converging a light beam having theinformation of the pattern, a spatial filter, and an imaging lens forforming the image of the pattern by the light beam having passed throughthe spatial filter. The minute-area light source causes an illuminationlight beam to be incident on an object surface having a pattern formedthereon as an object to be read. The spatial filter is disposed nearerto the objective lens than the paraxial image point of the image of thelight source. The spatial filter also has a shading region for shadingthe light beam for forming the image of the light source which iscontained in the light beam having passed through the objective lens.

[0021] According to yet another aspect of the present invention, thereis provided, a pattern reading apparatus including a minute-area lightsource, an objective lens, a spatial filter, an imaging lens, and animaging element disposed at the imaging position of the pattern imagefor reading the pattern. The objective lens causes the illuminationlight beam from the minute-area light source to be incident on an objectsurface having a pattern formed thereon as an object to be read andconverges the light beam reflected at the object surface. The spatialfilter is disposed nearer to the objective lens than the paraxial imagepoint of the light source formed through the objective lens forcapturing the scattered reflected component which is contained in thereflected light beam having passed through the objective lens. Theimaging lens forms an image of the pattern by the component havingpassed through the spatial filter.

[0022] According to yet another aspect of the present invention, thereis provided, a pattern reading apparatus using a Fourier conversionoptical system composed of a first lens, an object surface to be read, asecond lens, a spatial filter, and an imaging surface which are disposedalong the traveling direction of the light beam from a light source. Thespatial filter is disposed nearer to the second lens than the back focalpoint of the second lens.

[0023] According to yet another aspect of the present invention, thereis provided, a pattern reading apparatus for causing a light beamemitted from a light source to be incident on an object surface. Theobject surface has a pattern formed thereon as an object to be readthrough an objective lens. The pattern reading apparatus is also forconverging the light beam reflected at the object surface through theobjective lens as well as reading the image of the pattern by formingthe image by an imaging lens. Also provided is a tilt mechanism forsupporting the objective lens such that the objective lens is rotatableabout a rotation axis which is perpendicular to the optical axis of theobjective lens.

[0024] According to yet another aspect of the present invention, thereis provided, a pattern reading apparatus including a minute-area lightsource, an objective lens, a spatial filter, an imaging lens, an imagingelement, and a tilt mechanism. The objective lens causes theillumination light beam from the light source to be incident on anobject surface having a pattern formed thereon as an object to be readand converges the light beam reflected at the object surface. Thespatial filter captures the scattered reflected component which iscontained in the reflected light beam having passed through theobjective lens. The imaging lens images the image of the pattern by thecomponent having passed through the spatial filter. The imaging elementis disposed at the imaging position of the pattern image for reading thepattern. The tilt mechanism supports the objective lens to allow turningabout a turning axis which is perpendicular to the optical axis of theobjective lens.

[0025] According to yet another aspect of the present invention, thereis provided, a pattern reading apparatus for causing the illuminationlight beam emitted from a minute-area light source to be incident on anobject surface through a first lens. The object surface has a patternformed thereon as an object to be read. The pattern reading apparatushas a second lens that converges a light beam having the information ofthe pattern and causes the converging light beam to be incident on animaging lens. The pattern reading apparatus is for forming the image ofthe pattern by the imaging lens and reading the formed image. A tiltmechanism is provided for supporting the second lens to allow turningabout a turning axis which is perpendicular to the optical axis of thesecond lens.

[0026] According to yet another aspect of the present invention, thereis provided, a pattern reading apparatus for causing the illuminationlight beam emitted from a minute-area light source to be incident on anobject surface. The object surface has a pattern formed thereon as anobject to be read. The pattern reading apparatus has an objective lensfor converging a light beam having the pattern information. The patternreading apparatus also causes the converging light beam to be incidenton an imaging lens, which forms the image of the pattern, and reads theimage. The objective lens is disposed such that the light beamoriginating from a point of the object surface and emitted from theobjective lens is changed to a non-parallel light beam. The imaging lensand an imaging surface are made movable along the optical axis directionof the imaging lens in order to change magnification.

[0027] According to yet another aspect of the present invention, thereis provided, a pattern reading apparatus including a minute-area lightsource, an objective lens, a spatial filter, and an imaging element. Theobjective lens causes the illumination light beam from the light sourceto be incident on an object surface, having a pattern formed thereon asan object to be read, and converges the light beam reflected at theobject surface. The spatial filter is for capturing a scatteredreflected component which is contained in the reflected light beamhaving passed through the objective lens. The imaging element isdisposed at the imaging position of the pattern image for reading thepattern. The imaging lens and the imaging element are movable along theoptical axis direction of the imaging lens in order to changemagnification.

[0028] According to yet another aspect of the present invention, thereis provided, a pattern reading apparatus for causing the illuminationlight beam emitted from a minute-area light source to be incident on anobject surface, having a pattern formed thereon as an object to be read.The pattern reading apparatus also has an objective lens that convergesa light beam having the pattern information and causes the convergedlight source to be incident on an imaging lens. The imaging lens formsthe image of the pattern. The pattern reading apparatus is also forreading the image and includes an adjustment mechanism for adjusting theposition of the light source in a plane which is perpendicular to theprincipal beam of the illumination light beam.

[0029] According to yet another aspect of the present invention, thereis provided, a pattern reading apparatus for causing the illuminationlight beam emitted from a minute-area light source to be incident on anobject surface having a pattern formed thereon as an object to be read.An objective lens converges a light beam having the pattern information.The light beams that pass through the objective lens are incident on animaging lens through a spatial filter. The imaging lens forms the imageof the pattern. The image is also read. The spatial filter is a filterhaving a shading region for shading paraxial rays. The apparatusincludes an adjustment mechanism for adjusting the relative positionalrelationship between the position of the image of the light sourceformed by the objective lens and the shading region of the spatialfilter in the plane which crosses the optical axis of the imaging lens.

[0030] According to yet another aspect of the present invention, thereis provided, a pattern reading apparatus including a minute-area lightsource disposed to cause an illumination light beam to be obliquelyincident on an approximately flat object surface having a pattern formedthereon as an object to be read at a predetermined incident angle. Thepattern reading apparatus also includes an objective lens for converginga light beam having the information of the pattern, a spatial filterhaving a shading region for shading the portion of the reflected lightbeam from the object surface which has passed through the spatial filterand forms the image of the light source, and an imaging element. Theimaging element is for reading the image of the pattern formed by thelight beam having passed through the spatial filter. The line extendingfrom the principal plane of a lens interposed between the object surfaceand an imaging surface and having an imaging action, the line extendingfrom the imaging surface and the line extending from the object surfacecross each other on an approximately straight line.

[0031] According to yet another aspect of the present invention, thereis provided, a pattern reading apparatus including a minute-area lightsource for illuminating an object surface having a pattern formedthereon as object to be read. The pattern reading apparatus alsoincludes an objective lens for converging a light beam having thepattern information, a spatial filter and a shift mechanism. The spatialfilter has a shading region for shading the light beam, which forms theimage of the light source, of the light beam having passed through theobjective lens. The shift mechanism is for supporting the objective lensso as to allow parallel movement in a direction approximatelyperpendicular to the optical axis of the objective lens. The image ofthe pattern formed by the component having passed through the spatialfilter is read.

[0032] According to yet another aspect of the present invention, thereis provided, a pattern reading apparatus including a minute-area lightsource disposed such that an illumination light beam is caused to beincident on an object surface having a pattern formed thereon as aobject to be read without passing through a lens. The pattern readingapparatus also includes an objective lens for converging a light beamhaving the pattern information, a spatial filter, and an imagingelement. The spatial filter has a shading region for shading theportion, which forms the image of the light source, of the light beamhaving passed through the objective lens. The imaging element is forreading the image of the pattern formed by the light beam having passedthrough the spatial filter.

[0033] According to yet another aspect of the present invention, thereis provided, a pattern reading apparatus including an objective lensdisposed in confrontation with an object surface as a reflection surfacehaving a pattern formed thereon as an object to be read. The patternreading apparatus includes a minute-area light source disposed at aposition which is conjugate with the center of curvature of the objectsurface through the objective lens for illuminating the object surfacethrough the objective lens. The pattern reading apparatus also includesan imaging lens and an imaging element. The imaging lens is disposedfarther from the object surface than the light source with the opticalaxis thereof in coincidence with the objective lens. The imaging elementis for reading the image of the pattern which is reflected at the objectsurface and formed through the objective lens and the imaging lens.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0034]FIG. 1 shows an optical system of a pattern reading apparatusaccording to a first embodiment;

[0035]FIG. 2 is a plan view showing an example of a spatial filter;

[0036]FIG. 3 is a plan view showing another example of a spatial filter;

[0037]FIG. 4 shows a specific arrangement of an optical system accordingto the first embodiment;

[0038]FIG. 5 shows a first modification of the first embodiment;

[0039]FIG. 6 shows a second modification of the first embodiment;

[0040]FIG. 7 shows a third modification of the first embodiment;

[0041]FIG. 8 shows a fourth modification of the first embodiment;

[0042]FIG. 9 shows a fifth modification of the first embodiment;

[0043]FIG. 10(A) shows an optical system of a pattern reading apparatusaccording to a second embodiment;

[0044] FIGS. 10(B) and 10(C) show a modification of the optical systemof FIG. 10(A) for making magnification adjustable;

[0045] FIGS. 11(A) and 11(B) show an optical system of a pattern readingapparatus according to a third embodiment;

[0046] FIGS. 12(A) and 12(B) show an optical system of a pattern readingapparatus according to a fourth embodiment;

[0047] FIGS. 13(A), 13(B), and 13(C) show an optical system of a patternreading apparatus according to a fifth embodiment;

[0048]FIG. 14 shows an optical system of a pattern reading apparatusaccording to a sixth embodiment;

[0049]FIG. 15 shows the optical system of FIG. 14 in a developed form;

[0050]FIG. 16 shows a specific arrangement of an optical systemaccording to the sixth embodiment;

[0051] FIGS. 17(A) through 17(H) are spot diagrams showing the size ofan image of a light source calculated based on the specific arrangementof FIG. 16;

[0052]FIG. 18 is a front view of a specific mechanical arrangement of apattern reading apparatus including the optical system of FIG. 14;

[0053]FIG. 19 is a side view showing the apparatus of FIG. 18;

[0054]FIG. 20 illustrates the movement loci of an imaging lens and animaging element for adjusting magnification;

[0055]FIG. 21 illustrates an alternative arrangement for adjusting apinhole unit;

[0056] FIGS. 22(A) and 22(B) illustrate an alternative arrangement foradjusting a spatial filter;

[0057]FIG. 23 shows the arrangement of FIG. 22 as mounted;

[0058]FIG. 24 shows a modification of an optical system according to thesixth embodiment;

[0059] FIGS. 25(A) and 25(B) show an optical system of a pattern readingapparatus according to a seventh embodiment;

[0060]FIG. 26 shows an optical system of a pattern reading apparatusaccording to an eighth embodiment;

[0061]FIG. 27 is a plan view showing the arrangement of a shiftmechanism for moving an objective lens;

[0062]FIG. 28 is a side view of the shift mechanism of FIG. 27;

[0063]FIG. 29 is a plan view showing an alternative arrangement of theshift mechanism of FIG. 27;

[0064]FIG. 30(A) shows an optical system of a pattern reading apparatusaccording to a ninth embodiment;

[0065]FIG. 30(B) shows a modification of the optical system of FIG.30(A);

[0066]FIG. 31 shows an optical system of a pattern reading apparatusaccording to a tenth embodiment;

[0067]FIG. 32 shows a modification of the optical system of FIG. 31;

[0068]FIG. 33 shows an optical system of a pattern reading apparatusaccording to an eleventh embodiment;

[0069]FIG. 34 shows a modification of the optical system of FIG. 33;

[0070]FIG. 35 shows an optical system of a pattern reading apparatusaccording to a twelfth embodiment; and

[0071]FIG. 36 shows a modification of the optical system of FIG. 35.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0072] Embodiments of a pattern reading apparatus according to thepresent invention will be described below.

First Embodiment

[0073]FIG. 1 is a schematic view showing the arrangement of the patternreading apparatus according to a first embodiment. As shown in FIG. 1, asilicon wafer OR has a reflective surface 1 a on which a pattern (inthis case, a serial number) is formed by laser etching. Further, thepattern reading apparatus includes an optical system composed of anillumination unit 10, an objective lens 20 and a detection unit 30. Theobjective lens 20 is disposed so that the optical axis Ax thereof isperpendicular to the reflective surface 1 a. The illumination unit 10and the detection unit 30 are disposed approximately symmetrically withrespect to the optical axis Ax on opposite sides thereof.

[0074] The illumination unit 10 includes a lamp 11 such as a halogenlamp, or the like, and a pinhole plate 12 in which a pinhole 12 a isformed to permit a portion of the light beam emitted from the lightsource to pass therethrough to form a minute-area light source (theminute-area light source will also be referred to as a point lightsource). A diffusion plate 13 is interposed between the lamp 11 and thepinhole plate 12 to eliminate any effect due to an image of a filamentof the lamp 11.

[0075] The detection unit 30 includes a spatial filter 31, an imaginglens 32, and an imaging element 33, such as a CCD image sensor, or thelike. In the embodiment shown in FIG. 1, the detection unit 30 isdisposed on a line extending in a direction in which light from theminute-area light source will be specularly reflected from the surface 1a.

[0076] The objective lens 20 is designed such that the minute-area lightsource is conjugate with the center of curvature of an object surface tobe read. In this embodiment, since the surface 1 a is formed as a plane,the pinhole 12 a is disposed at a front focal point position (i.e., aposition on a plane which is perpendicular to the optical axis Ax of theobjective lens 20, and which includes a front focal point of theobjective lens 20) of the objective lens 20. A light beam emitted fromthe minute-area light source becomes a parallel light beam after passingthrough the objective lens 20 and obliquely illuminates the surface 1 aof the silicon wafer OR. The parallel light beam is scatteringly(diffusely) reflected at edges of the pattern and specularly reflectedat portions other than the edges.

[0077] The reflected light beam passes through the objective lens 20again, and becomes a converging light beam directed toward the detectionunit 30. The spatial filter 31 is disposed at a position where it isconjugate with the minute-area light source through the objective lens20, that is, at a back focal point position of the objective lens 20 inthe optical path between the imaging lens 32 and the objective lens 20(i.e., a position on a plane which is perpendicular to the optical axisAx of the objective lens 20, and which includes a back focal point ofthe objective lens 20). As a result, at the spatial filter 31, aspecularly reflected component of the light beam reflected from thesurface 1 a is converged to a beam spot approximately the same size asthe pinhole 12 a. As shown in FIGS. 2 and 3, the spatial filter 31 isprovided with a shading portion for shading the specularly reflectedcomponent. Specifically, as shown in, for example, FIG. 2, the spatialfilter 31 has a shading portion 31 a for covering the central portion ofthe pupil of the imaging lens 32 which corresponds to a range on whichthe specularly reflected light beam is incident and the right half ofthe pupil of the imaging lens 32. In another example, the spatial filter31 has, as shown in FIG. 3, a shading portion 31 b for covering only thecentral portion of the pupil of the imaging lens 32 which corresponds toa range on which the specularly reflected light beam is incident. In theabove examples, the spatial filter 31 may have a transparent glass plateas shown by broken lines, and the shading portion 31 a or 31 b is formedby a coating or the like.

[0078] The diffusely reflected component of the light beam reflected atthe surface 1 a (see FIG. 1), which passes through the spatial filter31, is incident on the imaging lens 32. A power and a position of theimaging lens 32 is designed such that the surface 1 a of the siliconwafer OR and the imaging element 33 are conjugate with respect to theimaging lens 32, and, thus, the image of the pattern is formed on theimaging element 33 by the scatteringly (diffusely) reflected component.The imaging element 33 converts the formed image of the pattern into anelectric signal and outputs the signal to an image processing apparatus(not shown). The image processing apparatus may display the image of thepattern on a display screen based on the input image signal and/oranalyze the content of the pattern using a character recognitionalgorithm.

[0079] In the example in FIG. 1, since the detection unit 30 is disposedon the line extending in the direction in which the specularly reflectedcomponent is reflected from the surface 1 a, if the spatial filter 31 isnot provided, the specularly reflected component will be incident on theimaging lens 32. Since the specularly reflected component does notinclude information of the pattern and has a strong intensity, if thespecularly reflected component is captured by the imaging element 33,the Signal to Noise (S/N) ratio of the information of the pattern islowered and it is difficult to detect the pattern. To cope with thisproblem, the S/N ratio of the information of the pattern is improved byremoving the specularly reflected component using the spatial filter 31and permitting the imaging element 33 to capture only the diffuselyreflected component so that it is easy to recognize and discriminate thepattern. Because the image formed on the imaging element is mainlyformed of a high frequency component of a spatial frequency of thecapture image, by suppressing the low frequency component thereof, theedge portion of the captured image of the pattern is actuallyemphasized.

[0080] The focal length of the imaging lens 32 is determined based on amagnification determined in accordance with the length of the pattern(i.e., the length of the serial number) and the size of the imagingsurface of the imaging element 33. Further, the focal length of theobjective lens 20 is determined based on the distance between thesurface 1 a and the imaging lens 32, where the distance between thesurface 1 a and the imaging lens 32 is set according to the focal lengthof the imaging lens 32 and the magnification.

[0081]FIG. 4 illustrates a design example of the pattern reading opticalsystem of a first embodiment. In this example, the imaging lens 32 has afocal length of 28 mm and the objective lens 20 has a focal length of250 mm. Further, the distance a1 from the optical axis Ax of theobjective lens 20 to the pinhole 12 a is about 60 mm, the distance b1from the pinhole 12 a to the surface 1 a of the silicon wafer OR isabout 300 mm and the distance c1 from the objective lens 20 to thesurface 1 a is about 50 mm. Assuming that the length of the pattern is 2cm, the image of the pattern is about 1.96 mm long on the imagingelement 33. Thus, the imaging element 33 may be, for example, ½ inch inlength.

[0082] FIGS. 5 to 7 show modifications of the optical system accordingto the first embodiment. In the modifications, the illumination unit 10,the imaging lens 32, and the imaging element 33 of the detection unit 30are the same as those of the first embodiment shown in FIG. 1.

[0083] In a first modification shown in FIG. 5, the imaging lens 32 isdisposed in a position at which the specularly reflected component willnot be incident thereon. In this way, the spatial filter 31 is notrequired. More particularly, in the example of FIG. 5, the imaging lens32 is disposed at a position which is farther from the optical axis Axof the objective lens 20 than in FIG. 1. Accordingly, only the diffuselyreflected component is incident on the imaging lens 32, and an image ofthe pattern, in which the edge portion is emphasized, is formed on theimaging element 33 without the spatial filter 31.

[0084] In a second modification, shown in FIG. 6, the illumination unit10 is disposed on the optical axis Ax of the objective lens 20 such thatan illumination light beam is incident on the surface 1 a of the siliconwafer OR at a right angle (i.e., along the optical axis Ax). In thismodification, a beam splitter 40 is disposed in the optical path betweenthe pinhole plate 12 and the objective lens 20 to separate the opticalpath of the illumination light beam emitted from the illumination unit10 from the optical path of the reflected light beam from the surface 1a.

[0085] The illumination light beam from the pinhole 12 a passes throughthe beam splitter 40 and the objective lens 20 to become a parallellight beam (also parallel with the optical axis Ax) that illuminates thesurface 1 a. The reflected light beam from the surface 1 a passesthrough the objective lens 20 again and becomes a converging light beam,a part of which is reflected at the beam splitter 40 toward the spatialfilter 31. A position of the spatial filter 31 is conjugate with theminute-area light source, similar to the first embodiment, and shadesthe specularly reflected component of the reflected light beam. Thediffusely reflected component passes through the spatial filter 31 andthe imaging lens 32 to form an image of the pattern on the imagingelement 33.

[0086] In a third modification, shown in FIG. 7, an objective lens iscomposed of an illumination lens (a first lens) 21 through which anillumination light beam passes and an objective lens (a second lens) 22through which the reflected light beam from the surface 1 a of thesilicon wafer OR passes. These lenses 21, 22 are disposed such that theoptical axes Ax1, Ax2 thereof cross each other at the surface 1 a of thesilicon wafer OR. The other arrangement and operation of the thirdmodification are the same as those of the first embodiment. It should benoted that in the third modification, the minute-area light source islocated at a focal point of the first lens 21, and the filter 31 islocated at a focal point of the second lens 22.

[0087]FIGS. 8 and 9 show further modifications of the first embodiment.These modifications have substantially the same structure as that of thesecond modification of FIG. 6. The modification shown in FIG. 8 is usedwhen an object to be read has a convex spherical surface 1 b and themodification shown in FIG. 9 is used when an object to be read has aconcave spherical surface 1 c.

[0088] In FIG. 8, an objective lens 20 makes a minute-area light sourceprovided by a pinhole 12 a conjugate with the center of curvature of theobject surface 1 b and causes an illumination light beam to be incidenton the object surface 1 b as a converging light beam that issubstantially perpendicular to the object surface 1 b. The illuminationlight beam is diffusely reflected at edges of the impressed pattern andspecularly reflected at portions other than the edges. These reflectedcomponents then become incident on the objective lens 20 again. Inparticular, the specularly reflected component passes through theobjective lens 20 along the same optical path as the illumination lightbeam.

[0089] In FIG. 9, an objective lens 23 makes a minute-area light sourceprovided by a pinhole 12 a conjugate with the center of curvature of theobject surface 1 c and causes an illumination light beam to be incidenton the object surface 1 c as a diverging light beam that issubstantially perpendicular to the object surface 1 c. The illuminationlight beam is diffusely reflected at edges of the impressed pattern andspecularly reflected at portions other than the edges. The reflectedcomponents are then incident on the objective lens 23 again. Inparticular, the specularly reflected component passes through theobjective lens 20 along the same optical path as the illumination lightbeam.

[0090] Note, the minute-area light source may also be a light emittingdiode rather than the combination of the halogen lamp 11 and the pinholeplate 12 used in the above and following embodiments and modifications.Because the light emitted by the light emitting diode is concentrated ata central portion, the light emitting diode may be suitably used as aminute-area light source that is near to a point light source.Alternatively, a combination of a lamp and an optical fiber may also beused to realize the minute-area light source. That is, the lamp and anincident end of the optical fiber may be located at a separatedposition, and the other end of the optical fiber can be used as theminute-area light source.

[0091] As described above, according to the first embodiment, because animage is formed using only the diffusely reflected component of thereflected light beam from an object (i.e., the specularly reflectedcomponent is prevented from reaching the imaging device), a pattern,such as a serial number, or the like, formed on a reflective surface,such as a silicon wafer, can be easily read. Therefore, the pattern canbe easily and accurately recognized by displaying the pattern or usingcharacter recognition techniques to decode the pattern. In particular,even if a pattern has deteriorated through processes such as etching,vapor deposition, and the like, it can be easily read.

Second Embodiment

[0092] FIGS. 10(A), (B) and (C) show optical systems for a patternreading apparatus according to a second embodiment and modificationsthereof. The second embodiment is an example of a filtering opticalsystem for detecting a pattern contained in a light-transmission-typeobject OT. A light beam emitted from a lamp (not shown) passes through apinhole plate 12 to form a minute-area light source, and the lightemitted from the minute-area light source is incident on the object OTthrough an illumination lens (first lens) 21. The light beam then passesthrough an objective lens (second lens) 22, a spatial filter 31, and animaging lens 32, to form an image of the object OT on an imaging surface33 a.

[0093] The spatial filter 31 has a shading region at a center thereoffor shading the portion of the light beam from the minute-area lightsource which has not been scattered by the object OT. In the secondembodiment, the spatial filter 31 is disposed nearer to the objectivelens 22 than a paraxial image point IM of the minute-area light source.The spatial filter 31 is similar to ones shown in FIG. 2 or FIG. 3.

[0094] In FIG. 10(A), the objective lens 22 is a Fourier transformationlens. In this case, the minute-area light source is located at the frontfocal point of the illumination lens 21 such that the object OT isilluminated by a parallel light beam. In addition, the object OT islocated at the front focal point of the objective lens 22 (the Fouriertransformation lens). The back focal point of the objective lens 22coincides with the front focal point of the imaging lens 32, and theimaging surface 33 a is located at the focal point of the imaging lens32.

[0095] In the first embodiment, the spatial filter 31 is located at theconjugate position of the minute-light source with respect to theobjective lens 20. In other words, in the first embodiment, the spatialfilter 31 is located at the paraxial image point IM of the minute-arealight source, that is, at the back focal point of the objective lens 20.However, if the objective lens includes aberrations such as sphericalaberration, coma, or curvature of field, the spread of the image of theminute-area light source is not reduced to a minimum at exactly theparaxial image point IM.

[0096] Thus, in the second embodiment, the spatial filter 31 is disposedat a position where the image of the minute-area light source is aminimum size after taking the effect caused by the spherical aberrationof the objective lens 22 and the effect resulting from the coma andcurvature of field caused by abaxial rays into consideration. With thisarrangement, the shading region may be made smaller than a region whichis located at the paraxial image point IM.

[0097] Specifically, the spatial filter 31 is disposed at the positionwhich satisfies the condition that the distance L from the final surfaceof the objective lens 22 to the spatial filter 31 is 0.60 fo<L<0.95 fo,wherein fo is the focal length fo of the objective lens 22. Because thesize of the image of the light source at a point within the range of0.60 fo<L<0.95 fo is smaller than the size at the paraxial image pointIM (L=fo), the shading region can be made smaller than that of the firstembodiment. Note, when the above arrangement is applied to an actualoptical system, it is preferable to determine a position where the sizeof the image of the minute-area light source is minimized by tracinglight rays. The spatial filter 31 is then placed at an appropriateposition in accordance with the shape of the image.

[0098]FIG. 10(B) shows a modification of the optical system in FIG.10(A) arranged to permit an adjustment of magnification. In thearrangement shown in FIG. 10(A), since the light beam emitted from apoint on the object OT becomes a parallel light beam after passingthrough the objective lens 22, as shown by a dotted line, magnificationcannot be changed by moving the imaging lens 32. In order to allowmagnification to be changed by moving the imaging lens 32, the opticalsystem in FIG. 10(B) is arranged such that a distance X from the objectOT to an objective lens 22 is set shorter than the focal length of theobjective lens 22. With this arrangement, a light beam from a point onthe object OT passes through the objective lens 22 to be a non-parallellight beam, and accordingly the magnification can be changed by movingan imaging lens 32 and an imaging surface 33 a.

[0099] In particular, it is preferable that the distance X satisfies thecondition 0<X< 0.7 fo where the focal length of the objective lens isfo. In this arrangement, because the paraxial image point IM where theimage of the light source is formed will also be closer to the objectOT, the spatial filter 31 can also be located nearer to the object OT ascompared with the optical system of FIG. 10(A). With this arrangement,the movable range of the imaging lens 32 is larger, providing a widervariable magnification range.

[0100] As a further modification, in FIG. 10(C), the position of aminute-area light source is located farther from an illumination lens 21than the front focal point of the illumination lens 21. With thisarrangement, since the illumination light beam emitted from theillumination lens 21 is a converging light beam, the position of theimage of the minute-area light source formed through an objective lens22 is formed nearer to the object OT, so that the movable range of theimaging lens 32 can be further increased allowing an even wider variablemagnification range.

[0101] According to the second embodiment and its modifications, sincethe spatial filter 31 is disposed at the position where the size of theimage of the minute-area light source formed by the objective lens issmallest, the area of the shading region of the spatial filter may bemade as small as possible, such that a bright image of the emphasizedimage of the pattern can be formed. Further, the magnification of theimage formed on the imaging device can be made variable.

Third Embodiment

[0102] FIGS. 11(A) and 11(B) show optical systems of the pattern readingapparatus according to a third embodiment. The third embodiment is anexample of a filtering optical system for detecting a pattern containedin a reflection type object to be detected similar to the firstembodiment.

[0103] In FIG. 11(A), a light beam from a lamp (not shown) is incidenton a pinhole plate 12 to form a minute-area light source. Light from theminute-area light source passes through an objective lens 20 and isobliquely incident on a reflection type object OR. The light beamreflected at the object OR is converged through the objective lens 20,passes through an imaging lens 32, and forms a pattern image of theobject OR on an imaging surface 33 a. In this embodiment, the objectivelens 20 is rotatable about a rotation axis Rx that is perpendicular to aplane of incidence and intersects the optical axis Ax of the objectivelens 20.

[0104] A portion of the illumination light beam which is incident on theobjective lens 20 from the pinhole plate 12 is reflected by theobjective lens 20 and may be incident on the imaging lens 32 as aghosting light. If the ghosting light overlaps the pattern image, thecontrast of the image is reduced and accordingly it may be difficult toread the pattern image. Thus, in this embodiment, the direction in whichthe ghosting light is reflected is changed by turning the objective lens20. In particular, because the direction of the ghosting light is verysensitive to the rotation of the objective lens 20, but the direction ofthe transmitted light beam is less sensitive to the rotation of theobjective lens 20, it is possible to change a position of the ghostinglight without substantially changing the position of the pattern image.In this embodiment, the spatial filter is not used. By reducing theghosting light, the contrast of the pattern image can be improved tomake it easier for a user to recognize the pattern image.

[0105]FIG. 11(B) shows a modification of the third embodiment, arrangedsuch that a light beam from a pinhole plate 12 passes through theobjective lens 20 and is perpendicularly incident on the object OR. Aportion of the reflected light beam from the object OR passes throughthe objective lens 20, is reflected at a beam splitter 40, and isincident on the imaging lens 32. As in the arrangement of FIG. 11(A),the objective lens 20 is rotatable about the rotation axis Rx that isperpendicular to a plane of incidence and intersects the optical axis Axof the objective lens 20. As a result, it is possible to adjust aposition of ghosting light so that the ghosting light does not overlapthe pattern image on the imaging surface 33 a.

[0106] The optical system of the third embodiment may also include aspatial filter similar to that of the first and second embodiments.

[0107] In this case, the effect of turning the objective lens can beused to control the position of the image of the minute-area lightsource formed on the spatial filter in addition to controlling thedirection of the ghosting light.

[0108] According to the third embodiment, when the reflection on thesurface of the objective lens prevents observation, and when thereflection surface of a reflection type object is tilted, a desiredfiltered output image can be obtained. The filtered output image can beobtained by changing the position where an image is formed by turningthe objective lens a predetermined angle about the turning axis of theobjective lens. The turning axis is perpendicular to the optical axisthereof.

Fourth Embodiment

[0109] FIGS. 12(A) and 12(B) show an optical system included in apattern reading apparatus according to a fourth embodiment. This is anexample of a filtering optical system for detecting a pattern containedin a light-transmission-type object, similar to the second embodiment.

[0110] A light beam emitted from a lamp (not shown) passes through apinhole plate 12 to form a minute-area light source. The light emittedfrom the minute-area light source passes through an illumination lens21, and is incident on a light-transmission-type object OT. The lightbeam passes through the object OT, through an objective lens 22, througha spatial filter 31, and forms an emphasized image of the object OT onan imaging surface 33 a. The spatial filter 31 has a shading region atthe center thereof for shading the light beam which forms the image ofthe light source. In this embodiment, as in the second embodiment, thespatial filter 31 is disposed nearer to the objective lens 22 than theparaxial image point IM of the light source. As in the third embodiment,the objective lens 22 is rotatable about a rotation axis Rx that isperpendicular to the optical axis thereof as shown in FIG. 12(A).

[0111]FIG. 12(A) shows a case in which the surface of the object OT isnot homogeneous. In this case, the shape of the image of the minute-arealight source may be deformed and may not conform with the shape of thepinhole. Thus, there is a possibility that the image of the minute-arealight source will not coincide with the shading region of the spatialfilter 31. By rotating the objective lens 22, the shape of the image ofthe minute-area light source is changed due to a change of coma and theobjective lens 22 may be rotated until the image of the minute-arealight source coincides with the shading region.

[0112]FIG. 12(B) shows a case in which the object OT is shaped andfunctions as a prism. In this case, the light beam forming the image ofthe minute-area light source is directed slightly upward from theoptical axis, and may not be shaded by the shading region of the spatialfilter 31. To cope with this problem, the objective lens 22 is rotated apredetermined acute angle counterclockwise (as shown in FIG. 12(B))about the rotation axis Rx. The rotation of the objective lens 22adjusts the position of the image of the minute-area light source, sothat the light beam which forms the image of the light source isappropriately shaded by the spatial filter 31.

Fifth Embodiment

[0113] FIGS. 13(A), 13(B), and 13(C) show an optical system for apattern reading apparatus according to a fifth embodiment. This is anexample of a filtering optical system for detecting a pattern containedin a light-transmission-type object, similar to the second embodiment.

[0114] A light beam emitted from a lamp (not shown) passes through apinhole plate 12 to form a minute-area light source. The light thenpasses through an illumination lens 21, and is incident on alight-transmission-type object OT. The light beam passes through theobject OT, through an objective lens 22, through a spatial filter 31,and forms an emphasized image of the object OT on an imaging surface 33a. The spatial filter 31 has a shading region at the center thereof forshading the portion of the light beam which forms the image of theminute-area light source. In this embodiment, the spatial filter 31 isdisposed nearer to the objective lens 22 than the paraxial imaging pointIM of the minute-area light source. In the fifth embodiment, the lamp(not shown) and the pinhole plate 12 are movable in a planeperpendicular to an optical axis as shown by an arrow A in FIG. 13(A).Further, the spatial filter 31 is also movable in a plane perpendicularto the optical axis as shown by an arrow B in FIG. 13(A).

[0115] In the optical system shown in FIG. 13(A), the pinhole plate 12is located at the front focal point of the illumination lens 21 and theobject OT is located at the front focal point of the objective lens 22(the Fourier transformation lens). The back focal point of the objectivelens 22 coincides with the front focal point of the imaging lens 32 andthe imaging surface 33 a is located at the focal point of the imaginglens 32.

[0116] FIGS. 13(B) and 13(C) show a case in which the object OT has aprism shape. In this case, since the light beam from the object OT isrefracted upward (in the view of FIGS. 13(A), 13(B), and 13(C)), theportion of the light beam that forms the image of the minute-area lightsource may not be shaded by the shading region of the spatial filter 31.In this case, as shown in FIG. 13(B), the light source and the pinholeplate 12 may be moved a predetermined amount upward to adjust theposition of the image of the minute-area light source such that theportion of the light beam which forms the image of the minute-area lightsource is shaded by the spatial filter 31. In FIG. 13(B), a solid lineindicates the case that a pinhole is located on the optical axis and adot-dash-line indicates the case that the pinhole plate is moved upward.Alternatively, as shown in FIG. 13(C), the spatial filter 31 may bemoved a predetermined amount upward (in the view of FIG. 13(C)) in aplane perpendicular to the optical axis in order to ensure that theposition of the image of the minute-area light source coincides with theshading region of the spatial filter 31.

[0117] According to the fifth embodiment, even if an object functions asa prism, the image of the light source can be caused to coincide withthe shading region of the spatial filter by adjusting the position ofthe light source or the position of the spatial filter. Theabove-described principle can be applied to a system using thereflection type object.

Sixth Embodiment

[0118] FIGS. 14 to 23 show a pattern reading apparatus according to asixth embodiment. The apparatus of the sixth embodiment is for areflection type object and provides all the features described in thesecond to the fifth embodiments, that is:

[0119] (1) the spatial filter 31 is placed nearer to the objective lens20 than the paraxial image point IM;

[0120] (2) the magnification is adjustable;

[0121] (3) the objective lens 20 is rotatable about the axis Rx which isperpendicular to a plane of incidence; and

[0122] (4) the lamp 11 and the pinhole plate 12 and/or the spatialfilter 31 are movable in a direction perpendicular to the optical axisof the objective lens 20.

[0123] As shown in a schematic view in FIG. 14, the optical system ofthe apparatus includes an illumination unit 10, an objective lens 20,and a detection unit 30. The objective lens 20 is disposed such that theoptical axis Ax thereof is perpendicular to the surface 1 a of a siliconwafer OR (i.e., a reflection surface) in a standard position. Theillumination unit 10 and the detection unit 30 are disposedapproximately symmetrically on opposite sides of the optical axis Ax ofthe objective lens 20 in the standard position.

[0124] The illumination unit 10 includes a lamp 11, a pinhole plate 12formed with a pinhole 12 a to form a minute-area light source, and adiffusion plate 13 provided between the lamp 11 and the pinhole plate12. The detection unit 30 includes a spatial filter 31, an imaging lens32, and an imaging element 33. In the example of FIG. 14, the detectionunit 30 is disposed on a line which extends in a direction in which aspecularly reflected component of light from the minute-area lightsource is reflected from the surface 1 a.

[0125] The light beam emitted from the lamp 11 passes through thepinhole 12 a, through the objective lens 20, and is incident on thesurface 1 a. The light beam is reflected at the surface 1 a, passesthrough the objective lens 20 again, and is incident on the spatialfilter 31. In this embodiment, the pinhole 12 a (the minute-area lightsource) is positioned at a front focal position of the objective lens 20(i.e., a position on a plane which is perpendicular to the optical axisAx of the objective lens 20, and which includes a front focal point ofthe objective lens 20) such that the light beam emitted from theobjective lens 20 is a parallel light beam and obliquely illuminates thesurface 1 a of the silicon wafer OR. The illumination light beam isdiffusely reflected at an impressed pattern portion of the surface 1 aand specularly reflected at portions other than the above.

[0126] The light beam reflected at the surface 1 a passes through theobjective lens 20 again, is transformed into a converging light beamdirected toward the detection unit 30 and reaches the spatial filter 31.The spatial filter 31 is disposed nearer to the objective lens 20 thanthe paraxial image position of the minute-area light source.

[0127] The diffusely reflected component, which has passed through thespatial filter 31, of the light beam reflected at the surface 1 a isincident on the imaging lens 32. The imaging lens 32 forms theemphasized image of the pattern impressed on the surface 1 a on theimaging element 33 by the diffusely reflected component. The imagingelement 33 converts the information of the emphasized image of thepattern into an electric signal and outputs the signal to an imageprocessing apparatus (not shown).

[0128] The objective lens 20 is rotatable about a rotation axis Rx, asshown by the arrow R in FIG. 14. In this embodiment, the rotation axisRx is parallel with a line where a plane, which is perpendicular to theprincipal beam Ax1 of the illumination light beam, crosses a plane whichis perpendicular the optical axis Ax2 of the imaging lens 32 (i.e., therotation axis Rx is perpendicular to a plane of incidence and crossesthe optical axis Ax). The objective lens 20 is rotatable with a range ofabout ±45 degrees from the standard position.

[0129] If a ghosting light, i.e., a reflection at the surface of theobjective lens 20, is incident on the imaging lens 32 and overlaps theposition of the pattern image on the imaging element 33, the objectivelens 20 is rotated so that the ghosting light does not overlap thepattern image.

[0130] When the distribution of the diffusely reflected light beam isuneven on the surface 1 a, there is a possibility that the shape of theimage of the light source is changed and displaced from the shadingregion 31 b of the spatial filter 31. In such a case, the shape of theimage of the light source can be changed by controlling coma by turningthe objective lens 20.

[0131] Further, the illumination unit 10 is adjustable in a direction,shown by the arrow S1, in a plane perpendicular to the optical axis Axin order to adjust the position of the image of the minute-area lightsource with respect to the shading region 31 b of the spatial filter 31.Still further, the spatial filter 31 is adjustable in a direction, shownby the arrow S2, in a plane that is perpendicular to the optical axisAx2 of the imaging lens 32.

[0132] In this way, if the surface 1 a is tilted, the position where theimage of the light source is formed can be adjusted to coincide with theshading region 31 b by adjusting the position of the illumination unit10 and/or the spatial filter 31.

[0133] Still further, the imaging lens 32 and the imaging element 33 arearranged such that they are movable along the optical axis Ax2 of theimaging lens 32, shown by the arrow S3, to change magnification. Inaddition, to permit the magnification to be changed by the movement ofthe imaging lens 32, the distance between the objective lens 20 and thesurface 1 a (object surface) is set to satisfy the condition 0<X<0.7 fo,where fo is the focal length of the objective lens 20. When thiscondition is satisfied, since a light beam emitted from a point on thesurface 1 a is not parallel after passing through the objective lens 20,the magnification can be changed by moving the imaging lens 32 along theoptical axis Ax2.

[0134]FIG. 15 shows an expanded optical path of the optical system ofFIG. 14. The light beam emitted from the pinhole plate 12, is collimatedto be a parallel light beam by the objective lens 20, reflected at thesurface 1 a (passes therethrough in FIG. 15), is incident on theobjective lens 20 again, passes through the spatial filter 31 as aconverging light beam, passes through the imaging lens 32, and forms animage of the pattern on the imaging element 33. The optical system inFIG. 14 is fundamentally equivalent to the optical system of the secondembodiment shown in FIG. 10(B) except with respect to the incidentdirection of the light beam and the transmission/reflectioncharacteristics of the object. That is, in the example in FIG. 14, thesurface 1 a is nearer to the objective lens 20 than the focal pointthereof and a light beam from a point on the surface 1 a is incident onthe imaging lens 32 not as a parallel light beam but as divergent light.

[0135]FIG. 16 shows a design example when it is assumed that the lengthof a pattern to be read is 2 cm and the size of the imaging surface ofan imaging element is ½ inch across a diagonal. In this example, theimaging lens 32 has a focal length of 50 mm and the objective lens 20has a focal length fo of 220 mm. Further, a distance b from the pinhole12 a to the surface 1 a of the silicon wafer OR is about 270 mm, adistance c from the objective lens 20 to the surface 1 a is about 50 mm,and a distance L from a final surface of the objective lens 20 to thespatial filter 31 is about 190 mm. Therefore, the condition 0.60fo<L<0.95 fo, described above, is approximately 130 mm<L<210 mm in thisexample. Further, the condition 0<X<0.7 fo, also described above, is0<X<154 mm.

[0136] FIGS. 17(A) through 17(H) are spot diagrams showing the shape ofthe image of a minute-area light source calculated based on the modelshown in FIG. 16, that is, the distribution of the light beam from thesurface 1 a which constitutes the specularly reflected component, atvarious distances DF from the paraxial image point of the minute-arealight source. The distance DF is 0 at the paraxial image point and aminus sign represents a position closer to the objective lens 20. In theexample, since the size of the image of the minute-area light source isminimized about 30 mm or 40 mm closer to the objective lens from theparaxial image point, the disposition of the spatial filter in thisrange permits the specularly reflected component to be shaded by a smallshading region so that a maximum possible quantity of light can be usedto form a bright image of the pattern.

[0137] Next, a specific mechanical arrangement of an apparatus includingthe optical system shown in FIG. 14 is described with reference to FIGS.18 and 19. Note, as shown in FIG. 18, a coordinate system x, y, z isdefined in which the x-axis is parallel with the optical axis Ax of theobjective lens 20 at the standard position. Further, the principal beamAx1 of an illumination light beam and the optical axis Ax2 of an imaginglens are contained in an x-z plane.

[0138] The pattern reading apparatus of the sixth embodiment includes abase frame 100 on which a silicon wafer (i.e., an object to beinspected) is placed at a reference position T, shown by adot-dash-line, and a movable frame 200 which is disposed on the baseframe 100, is supported by bearings 101 so as to slide in the directiony with respect to the base frame 100.

[0139] The movable frame 200 is moved by a frame drive mechanism 210(shown in FIG. 19). As shown in FIG. 19, the frame drive mechanism 210includes a ball screw 211 which is disposed to a screw support portion102, secured to the base frame 100 in such a manner that the rotation ofthe ball screw 211 can be adjusted, and a threading member 212 which issecured to the horizontal support plate 201 (parallel with a y-z plane)of the movable frame 200. The ball screw 211 includes an operation knob211 a on an outer side thereof for operation by an inspector and a screwportion 211 b formed on an inner side, i.e., a portion projecting towardthe movable frame 200. The screw portion 211 b of the ball screw 211 isscrewed into a screw hole provided in the threading member 212. Thus,when the ball screw 211 is rotated, the movable frame 200 slides in thedirection y.

[0140] The movable frame 200 is provided with a horizontal support plate201 and a tilt mechanism 220 is disposed to the horizontal support plate201 for rotatably supporting the objective lens 20. A column 202 extendsperpendicularly from the horizontal support plate 201 and a verticalsupport plate 203 (parallel with the x-z plane) is secured to the column202. The vertical support plate 203 supports an optical fiber 11 d and apinhole plate 12 which constitute a minute-area light source and animaging unit 320. The imaging unit 320 includes a lens barrel 32Ahousing the imaging lens 32 and a CCD unit 33A housing the imagingelement 33. Optical path holes 100 a, 201 a, 221 a are formed in thehorizontal support plate 201, the base frame 100, and the base plate221, respectively, such that the light beam from the lamp 11 may passthrough to the silicon wafer to allow the reflected light beam from thesilicon wafer to pass to the imaging unit 320.

[0141] The tilt mechanism 220 includes the column 202, the base plate221, and a bearing unit 222 (see FIG. 19). The base plate 221 isdisposed between the column 202 and the support member 204 (see FIG.18). The support member 204 extends from the horizontal support plate201 in parallel with the column 202. The bearing unit 222 extends belowthe base plate 221. The objective lens 20 is accommodated in a lensframe 223 having a rotation shaft 223 a extending in the direction y.The lens frame 223 is rotatably mounted to the bearing unit 222 throughthe turning shaft 223 a. Opposite ends of the turning shaft of the lensframe 223 project from the bearing unit 222. A follower pulley 224 issecured to the end of the turning shaft projecting toward the framedrive mechanism 210 and a rotary plate 225 of an encoder is secured tothe other end thereof.

[0142] A lens drive motor 226 is mounted on the base plate 221 of thetilt mechanism 220 and a timing belt 227 is stretched between a drivepulley 226 a secured to the rotary shaft of the motor 226 and thefollower pulley 224. The encoder is composed of the rotary plate 225mounted on a rotary shaft and a photo interrupter 228 composed of alight emitting element (not shown) and a light receiving element (notshown) disposed across the rotary plate 225. The rotary plate 225 has aslit (not shown) formed radially thereon and is adjusted such that whenthe objective lens 20 is set at the standard position, a light beamemitted from the light emitting element of the photointerrupter 228 isdetected by the light receiving element through the slit. As describedabove, the standard position of the objective lens 20 in this example isa position where the optical axis Ax of the objective lens 20 is setperpendicular to an ideal object surface (a flat surface).

[0143] The lamp 11 is composed of a halogen lamp 11 a, an infrared-raycut filter 11 b for reducing a heating component of the converging lightbeam emitted from the halogen lamp 11 a, a negative lens 11 c for makingthe converging light beam an approximately parallel light beam, and anoptical fiber 11 d. A pinhole unit 120 includes the pinhole plate 12, amounting plate portion 122 formed perpendicularly to the pinhole plate12, and a holding unit 121 for holding the exit end of the optical fiber11 d. The pinhole unit 120 is mounted on the vertical support plate 203by bolts 123, 123 through the mounting plate portion 122. Securinggrooves 124, 124 formed in the mounting plate portion 122 extend in aplane which is perpendicular to the axis Ax1 of the illumination lightbeam, so that, by loosening the bolts 123, the unit 120 is easilymovable in a direction perpendicular to the axis Ax1, i.e., closer tothe imaging unit 320 or away from the imaging unit 320.

[0144] In this example, the optical fiber 11 d is a commerciallyavailable optical fiber having a diameter of about 5 mm. Thus, a pinhole12 a is used reduce the light beam from the optical fiber 11 d to aminute-area light source, however, if the optical fiber 11 d has adiameter of 1 mm to 2 mm, the pinhole 12 a is not necessary. Further, ifthe density of the light beam emitted from the end surface of theoptical fiber 11 d is uneven, it is preferable to provide a diffusionplate (not shown) between the end surface of the fiber 11 d and thepinhole 12 a.

[0145] The spatial filter 31 is secured to a filter holder 130 which isthen secured to the vertical support plate 203. The spatial filter 31includes a shading region at the center thereof, similar to that shownin FIG. 3. The spatial filter 31 is arranged at a position which isnearer to the objective lens 20 than the paraxial image point such thatthe size of the image of the light source formed by the objective lens20 is minimized.

[0146] The imaging unit 320 is also mounted on the vertical supportplate 203. The vertical support plate 203 is formed with a slot 205 in adirection parallel to the optical axis Ax2 of the imaging lens 32. Theslot 205 includes a first wide step portion 205 a formed on the imagingunit 320 side to a depth approximately half a plate thickness and asecond narrow step portion 205 b formed at the center in the widthdirection of the first step portion 205 a passing through the verticalsupport plate 203 from the first step portion 205 a. The imaging unit320 includes two mounting arms 322 formed thereto that are each insertedthrough a washer 323 provided in the first step portion 205 a of theslot 205 and then secured by bolts 321 screwed into the ends of each ofthe arms 322 from the opposite side of the vertical support plate 203.The washers 323 have a diameter smaller than the first step portion 205a and larger than the second step portion 205 b. With the abovearrangement, the imaging unit 320 is movable in the direction of theoptical axis Ax2 of the imaging lens 32. Further, the imaging lens 32can also be adjusted in the optical axis direction by a lens barreladjustment mechanism (not shown). Thus, in the embodiment, themagnification can be changed by one or both of the above twoadjustments.

[0147] In the apparatus of the sixth embodiment, since the position T isdetermined so that the silicon wafer is located closer to the objectivelens 20 than the focal point thereof, the reflected light beam from apoint on the surface of the silicon wafer is incident on the imaginglens 32 as a divergent light beam. As a result, according to thearrangement of the sixth embodiment, the magnification can be changed bymoving the imaging lens 32 in the optical axis direction. However, whenthe imaging lens 32 is moved to change the magnification, the focusingstate of the pattern to the imaging element 33 is also changed.

[0148] Thus, in the embodiment, to change the magnification whilemaintaining the focusing state of the pattern, the positions of theimaging lens 32 and the imaging element 33 are adjusted, respectively,so that they move along the loci shown in FIG. 20. In FIG. 20, themagnification gradually increases from the upper side to the lower sideand the positions of the imaging lens 32 and the imaging element 33 areindicated for the case when the surface of the silicon wafer (objectsurface) is not moved. That is, when the imaging lens 32 and the imagingelement 33 are located at positions where an arbitrary horizontalstraight line crosses the respective locus lines, respectively, apattern image which is formed on the imaging element 33 is brought intofocus at the related magnification.

[0149] When a pattern is to be read, the silicon wafer is placed at thereference position T shown by the dot-dash-line in FIGS. 18 and 19, inparticular, for a pattern of characters, symbols, and the like, thesilicon wafer is positioned so that the lengthwise direction of thepattern coincides with the direction y. After the silicon wafer ispositioned, the halogen lamp 11 a is lit. The light beam emitted fromthe optical fiber 11 d passes through the pinhole 12 a to form anillumination light beam that is obliquely incident on the objective lens20 and is transmitted to the silicon wafer (object surface).

[0150] The illumination light beam is reflected at the surface of thesilicon wafer, passes through the objective lens 20 again, and isdirected toward the imaging unit 320. The portion of the reflected lightbeam corresponding to the image of the light source, that is, thespecularly reflected light beam, is shaded by the shading region 31 b ofthe spatial filter 31, and other portions of the reflected light beam,that is, the diffusely reflected light beam, is incident on the imagingunit 320. An emphasized image of the pattern is formed on the imagingelement 33 by the imaging lens 32 and an image signal is read by drivingthe imaging element 33.

[0151] If ghosting light, which is caused by surface reflection at theobjective lens 20, overlaps the pattern image, the tilt of the objectivelens 20 is changed by controlling the lens drive motor 226.

[0152] Further, if the surface of the silicon wafer is not flat, forexample, if it has a prism shape, the pinhole unit 120 is moved closerto or away from the imaging unit 320 in the plane perpendicular to theprincipal beam Ax1 of the illumination light beam so that the image ofthe light source is formed on the shading region of the spatial filter31.

[0153] Note, although the tilt of the silicon wafer itself may also beadjusted to adjust the position of the image of the light source, theapparatus according to the present embodiment is arranged for adjustingthe pinhole unit 120. In particular, the provision of a tiltingmechanism for adjusting each object to be inspected such that thereflected direction of a light beam is accurately controlled wouldrequire high sensitivity and a complicated mechanism such that theapparatus would be more expensive.

[0154]FIG. 21 shows an alternative arrangement for adjusting theposition of the pinhole unit 120.

[0155] In this arrangement, a rail member 125 is provided with a guidegroove 125 a extending in the direction z and is secured to a movableframe (not shown). A pinhole unit 120 a, to which a pinhole plate 12 andthe exit end of an optical fiber 11 d are secured, is mounted so as tomove in the direction z along the guide groove 125 a. If thisarrangement is combined with the arrangement of FIGS. 18 and 19, thepinhole unit 120 a may be moved in a plane which is perpendicular to theaxis Ax1 of the illumination light beam according to the arrangementshown in FIG. 18 or may be moved in a plane perpendicular to the opticalaxis Ax of the objective lens 20 according to the arrangement shown inFIG. 21.

[0156] In a further alternative arrangement, the position of the spatialfilter 31 may be made adjustable for the purpose of adjusting therelative positional relationship between the image of the light sourceand the shading region 31 b of the spatial filter 31. FIGS. 22(A),22(B), and 23 show a mechanism for adjusting the position of the spatialfilter 31. FIG. 22(A) is a plan view of a movable frame, FIG. 22(B) is asectional view taken along the line B-B of FIG. 22(A), and FIG. 23 is aplan view showing the movable frame assembled to fixed rails.

[0157] As shown in FIGS. 22(A), 22(B), and 23, the rectangular movableframe includes two rail members 131 a, 131 a, each having a C-shapedcross section with the openings thereof facing inward, disposed parallelto each other and separated by a predetermined distance. Two beammembers 132 a, 132 b are disposed between the rail members 131 a, 131 aat positions near opposite ends thereof. The spatial filter 31 isinserted into the C-shaped openings of the rail members 131 a, 131 b andfixed by presser screws 133. Guide pins 134 are provided on the movableframe at four comers thereof and are engaged with two guide grooves 136a, 136 b formed on fixed rails 135 a, 135 b.

[0158] According to the arrangement of FIG. 23, the spatial filter 31 ismovable in the direction Y with respect to the movable frame and themovable frame is further movable in the direction Z by sliding on thefixed rails 135 a, 135 b. Therefore, the position of the spatial filter31 can be adjusted in a Y-Z plane and the position of the shading region31 b of the spatial filter 31 can be adjusted so that the image of thelight source is formed on the shading region 31 b.

[0159]FIG. 24 shows a modification of the optical system according tothe sixth embodiment. As shown in FIG. 24, the arrangement of theillumination unit 10 and the imaging lens 32 and the imaging element 33of the detection unit 30 are the same as those shown in FIG. 14. In themodification of FIG. 24, the illumination unit 10 is disposed at aposition where an illumination light beam is perpendicularly incident onthe surface 1 a of the silicon wafer OR. That is, a pinhole plate 12having a pinhole 12 a for forming a minute-area light source is disposedon the optical axis Ax of an objective lens 20 which is perpendicular tothe surface 1 a. A beam splitter 40 is disposed in the optical pathbetween the pinhole plate 12 and the objective lens 20 to separate theoptical path of the illumination light beam emitted from theillumination unit 10 from the optical path of the reflected light beamfrom the surface 1 a.

[0160] The illumination light beam from the pinhole 12 a passes throughthe beam splitter 40 and the objective lens 20 to become a parallellight beam (also parallel with the optical axis Ax) that illuminates thesurface 1 a. The reflected light beam from the surface 1 a passesthrough the objective lens 20 again and becomes a converging light beam,a portion of which is reflected at the beam splitter 40 toward thespatial filter 31. The spatial filter 31 is at a position nearer to theobjective lens 20 than a position which is conjugate with theminute-area light source and shades the specularly reflected componentof the reflected light beam from the surface 1 a. The diffuselyreflected component passes through the spatial filter 31 and the imaginglens 32 to form an image of the pattern on the imaging element 33.

Seventh Embodiment

[0161] FIGS. 25(A), (B) show an optical system included in a patternreading apparatus according to a seventh embodiment. The seventhembodiment is an example of a filtering optical system for detecting apattern contained in a light-transmission-type object OT.

[0162] A light beam emitted from a light source (not shown) passesthrough a pinhole plate 12 to form a minute-area light source and isincident on the object OT through an illumination lens 21. The lightbeam then passes through an objective lens 22, a spatial filter 31, andan imaging lens 32, to form an image of the object OT on an imagingsurface 33 a.

[0163] The spatial filter 31 has a shading region at the center thereoffor shading a portion of the light beam which forms the image of thelight source and is disposed nearer to the objective lens 22 than theparaxial imaging surface IM of the light source. According to thearrangement, the image of the pattern on the object OT is formed on theimage surface 33 a only by the scattered component of light from theobject OT.

[0164] In the optical system shown in FIG. 25(A), the pinhole plate 12is located at the front focal point of the illumination lens 21 and theobject OT is illuminated by a parallel light beam. The object OT islocated at the front focal point of the objective lens 22 (the Fouriertransformation lens). The back focal point of the objective lens 22coincides with the front focal point of the imaging lens 32 and theimaging surface 33 a is located at the focal point of the imaging lens32.

[0165]FIG. 25(B) shows the case that the object OT has a prism shape,such that a light beam is deflected upward (in the view of FIG. 25(B)),that is, the object OT is formed as a wedge which is thinner at thelower edge (in the view of FIG. 25(B)). In this case, if the objectivelens 22 is left in the state shown in FIG. 25(A), the image of the lightsource deviates upward and the portion of the light beam forming theimage of the light source may not be shaded by the shading region of thespatial filter 31. To cope with this problem, as shown in FIG. 25(B),the objective lens 22 is arranged to be movable a predetermined distancein a direction opposite to the direction in which the light beam isrefracted by the wedge, that is, to be moved downward in the view ofFIG. 25(B). The deviation of the image of the light source caused by theprism shape of the object OT can be compensated for by movement of theobjective lens 22. As a result, the portion of the light beam whichforms the image of the light source can be appropriately shaded by thespatial filter 31.

[0166] Specifically, in the example in FIG. 25(B), if it is assumed thatthe object OT is thinner at the lower side, has an angle (the apex ofthe prism) of 20 minutes, and a refractive index of 1.5 and theobjective lens 22 has a focal length of 200 mm, the deviation of theimage of the light source caused by the effect of the wedge can becompensated by parallel movement of the objective lens 20 by about 300μm in a direction which is opposite to the direction in which the objectOT is thinner (downward), that is, in the direction in which the lightbeam is deflected by the wedge.

Eighth Embodiment

[0167]FIG. 26 to FIG. 28 show the arrangement of a pattern readingapparatus according to an eighth embodiment. The eighth embodiment is anexample in which the principle of the parallel movement of the objectivelens in the seventh embodiment is applied to an optical system fordetecting a pattern contained in a light-reflection-type object.

[0168] As shown in FIG. 26, the optical system of the apparatus iscomposed of an illumination unit 10, an objective lens 20, and adetection unit 30. The objective lens 20 is a bi-convex lens and isdisposed such that the optical axis Ax thereof is perpendicular to thesurface 1 a of a silicon wafer OR (reflection surface). The illuminationunit 10 and the detection unit 30 are disposed approximatelysymmetrically on opposite sides of the optical axis Ax of the objectivelens 20. As shown in FIG. 26, in this embodiment, the optical axis Ax1of the illumination unit 10 and the optical axis Ax2 of the detectionunit 30 cross the optical axis Ax of the objective lens at the surface 1a. The objective lens 20 is supported by a shift mechanism 400 so as tobe movable perpendicular to the optical axis Ax of the objective lens 20as well as in parallel with a direction X which is parallel with a planecontaining the optical axes Ax1, Ax2 (which coincides with the papersurface in FIG. 26). The amount of parallel movement M of the objectivelens 20 should approximately satisfy the following condition:

D/2<M<D/2,

[0169] where D is the diameter of the objective lens 20.

[0170] The illumination unit 10 includes a lamp 11 such as a halogenlamp, or the like, and a pinhole plate 12 in which a pinhole 12 a isformed to permit a portion of the light beam emitted from the lightsource to pass therethrough to form a minute-area light source. Adiffusion plate 13 is interposed between the lamp 11 and the pinholeplate 12 to eliminate any effect due to an image of a filament of thelamp 11.

[0171] The detection unit 30 includes a spatial filter 31, an imaginglens 32, and an imaging element 33, such as a CCD image sensor, or thelike. In the embodiment shown in FIG. 26, the detection unit 30 isdisposed on a line extending in a direction in which light from thelight source will be specularly reflected from the surface 1 a.

[0172] A light beam emitted from the lamp 11 becomes a parallel lightbeam after passing through the objective lens 20 and obliquelyilluminates the surface 1 a of the silicon wafer OR. In particular, thepinhole 12 a is disposed at the front focal position of the objectivelens 20 (i.e., a position on a plane which is perpendicular to theoptical axis Ax of the objective lens 20, and which includes a frontfocal point of the objective lens 20). The parallel light beam isdiffusely reflected at edges of the pattern and specularly reflected atportions other than the edges.

[0173] The reflected light beam from the surface 1 a passes through theobjective lens 20 again, and becomes a converging light beam directedtoward the detection unit 30. The spatial filter 31 is disposed betweenthe imaging lens 32 and the objective lens 20 at a position nearer tothe objective lens 20 than the image of the light source formed by theobjective lens 20. Thus, only the diffusely reflected component thatpasses through the spatial filter 31, is incident on the imaging lens32, and the image of the pattern impressed on the surface 1 a is formedon the imaging element 33 by the diffusely reflected component. Theimaging element 33 converts the image of the pattern into an electricsignal and outputs the signal to an image processing apparatus (notshown).

[0174] The parallel movement of the objective lens 20 is effective toprevent the effect of the ghosting light in the reflection-type system,similar to the eighth embodiment, in addition to compensate for theeffect due to the wedge-shaped object as described for the fifthembodiment.

[0175] In particular, when ghosting light, which is made by reflectionat the surface of the objective lens 20, is incident on the imaging lens32 and overlaps the position of the image pattern on the imaging element33, it is difficult to read the image pattern because the contrastthereof is lowered. In such a case, by adjusting the objective lens 20,by the parallel movement thereof, so that the ghosting light does notoverlap the image pattern, the contrast is not lowered and the patterncan be correctly read. Further, if the surface 1 a is tilted, forexample if the silicon wafer OR has a wedge shape, the position wherethe image of the light source is formed can be adjusted to coincide withthe shading region of the spatial filter 31 by parallel movement of theobjective lens 20.

[0176] When the objective lens is moved to lower the effect of theghosting light, at least the surface of the objective lens 20, where theghosting light is made, must be a curved surface. When both the surfacesof the objective lens 20 are curved as in the case of FIG. 26, bothghosting light caused by the surface reflection arising at the lenssurface on the side of the light source and ghosting light due to theinner surface reflection caused at the lens surface on the side of thesilicon wafer OR can be eliminated by the parallel movement of theobjective lens 20.

[0177]FIG. 27 is a plan view showing the arrangement of the shiftmechanism 400 for parallel movement of the objective lens 20 and FIG. 28is a side view thereof. The objective lens 20 is supported by aflat-plate-shaped lens holder 410. The lens holder 410 is guided by apair of guide rails 420, 421 and is movable in a direction X. The guiderails 420, 421 are coupled with each other by bridge members 430, 431 atends thereof. Thus, a rectangular frame is formed by the guide rails420, 421 and the bridge members 430, 431.

[0178] A pair of tension springs 440, 441 are interposed between thelens holder 410 and the bridge member 430 such that the lens holder 410is drawn towards the bridge member 430. Further, a micrometer head 450is fixed to the center of the bridge member 430 and an end of themicrometer head 450 abuts the lens holder 410 such that the position ofthe lens holder 410, that is, the position of the objective lens 20 maybe adjusted by rotating the micrometer head.

[0179] In particular, the micrometer head 450 may be rotated such thatthe lens holder 410 moves downward in the view of FIG. 27 against theurging force of the springs 440, 441, or such that the lens holder 410moves upward in the view of FIG. 27 by being pulled by the springs 440,441. Thus, the objective lens 20 can be set to an optimum position, thatis, a position where the image of the light source coincides with theshading portion of the spatial filter 31 and ghosting light is notincident on the imaging element 33 by adjusting the micrometer head 450while observing an image formed on the imaging element 33.

[0180] If the silicon wafer OR also has a tilt or wedge shape in adirection perpendicular to the paper surface of FIG. 26 (direction Y),it is preferable to also adjust the objective lens 20 in the directionY. FIG. 29 is a plan view showing an alternative shift mechanism bywhich the objective lens 20 may also be adjusted in the direction Y,perpendicular to the optical axis Ax of the objective lens 20 as well asperpendicular to the direction X. The shift mechanism includes aY-direction shift mechanism 500 and the X-direction shift mechanism 400shown in FIG. 27.

[0181] The Y-direction shift mechanism 500 includes a pair of guiderails 520, 521 for guiding the X-direction shift mechanism 400 forparallel movement and bridge members 530, 531 for coupling the guiderails 520, 521 at the ends thereof to form a rectangular frame. A pairof tension springs 540, 541 are interposed between the bridge member 530and the guide rail 421 of the x-direction shift mechanism 400 such thatthe X-direction shift mechanism 400 is drawn towards the bridge member530. Further, the bridge member 530 is provided with a micrometer head550, an end of which is abutted against the guide rail 421.

[0182] Similar to the above, the micrometer head 550 may be adjustedsuch that the x-direction shift mechanism 400 is moved against theurging force of the springs 540, 541 or such that the x-direction shiftmechanism 400 is moved by being pulled by the springs 540, 541. Thus,according to the arrangement of FIG. 29, if the silicon wafer OR has atilt or wedge component in any direction, the objective lens 20 can beset to a position where the image of the light source coincides with theshading portion of the spatial filter 31 and ghosting light is notincident on the imaging element 33 by adjusting the objective lens 20 inthe X-Y direction.

[0183] Note that the amount of shift of the objective lens with respectto the angle of the silicon wafer OR is different depending upon adirection in which the angle is formed. For example, if the siliconwafer OR is tilted 1 degree in the direction X, a light beam isangularly changed only in the direction X and the amount of change isabout 2 degrees, whereas if the silicon wafer OR is tilted 1 degree inthe direction Y, the light beam is angularly changed 1.4 degrees in thedirection Y and angularly changed by a small amount in the direction X.Therefore, when the silicon wafer OR is tilted in the direction X, itsuffices to shift the objective lens 20 in only the direction X,however, when the silicon wafer OR is tilted in the direction Y, theobjective lens 20 should be adjusted in both the directions X and Y.

Ninth Embodiment

[0184]FIG. 30(A) shows an optical system included in a pattern readingapparatus according to a ninth embodiment. The ninth embodiment is anexample of a filtering optical system for detecting a pattern containedin a light-transmission-type object.

[0185] A light beam emitted from a light source (not shown) passesthrough a pinhole plate 12 to form a minute-area light source and isdirectly incident on the object OT without passing through a lens. Thelight beam passes through the object OT, through an objective lens 22,and through a spatial filter 31, to form an image of the object OT on animaging surface 33 a. The spatial filter 31 has a shading region at thecenter thereof for shading the portion of the light beam which forms theimage of the light source, i.e., a portion of the light beam that is notscattered by the object OT. The spatial filter 31 is disposed nearer tothe objective lens 22 than the paraxial image point IM of the lightsource.

[0186] In the optical systems for reading the pattern of thelight-transmission-type object in the above embodiments, an illuminatinglens is interposed between the pinhole plate 12 and the object OT, anobjective lens is interposed between the object OT and the spatialfilter, and an imaging lens is interposed between the spatial filter andthe imaging element. Thus, there are three lenses. On the other hand, inthe optical system shown in FIG. 30(A), since the light beam is directlyincident on the object OT, only two lenses are required, such that thecost of the optical system can be reduced.

[0187] The optical system in FIG. 30(B) shows a modification of theninth embodiment in which the imaging lens 32 of FIG. 30(A) is notrequired. In this case, the objective lens 22 is provided with animaging power for forming the image of a pattern on the imaging element33. According to the arrangement shown in FIG. 30(B), only one lens isincluded in the optical system, such that the cost of the optical systemcan be further reduced from that of the arrangement in FIG. 30(A).

Tenth Embodiment

[0188]FIG. 31 shows an arrangement of a pattern reading apparatusaccording to a tenth embodiment. The tenth embodiment is an example inwhich the principle of reducing the number of lenses of the ninthembodiment is applied to an optical system for detecting a patterncontained in a light-reflection-type object.

[0189] In an apparatus for reading a pattern on a light-reflection-typeobject, a single objective lens can act as both an illumination lensbetween a light source and an object and an objective lens between theobject and a spatial filter, however, there are some problems. Forexample, when an incident light beam is obliquely incident on an objectsurface, an objective lens with a large diameter is required. Further,when the light beam is perpendicularly incident on the object surface, abeam splitter is necessary to separate a reflected light beam and thequantity of light which reaches an imaging element is lowered to abouthalf that when the light beam is obliquely incident.

[0190] As shown in FIG. 31, the optical system of the apparatus includesan illumination unit 10, an objective lens 23 and a detection unit 30.The illumination unit 10 and the detection unit 30 are positionedsymmetrically with respect to the normal of a surface 1 a so that when alight beam passing through the center of a pinhole which coincides withthe optical axis Ax1 of the illumination unit 10 is specularlyreflected, the light beam coincides with the optical axis Ax2 of thedetection unit 30.

[0191] The illumination unit 10 includes a lamp 11, a pinhole plate 12with a pinhole 12 a (a minute-area light source), and a diffusion plate13. The detection unit 30 includes a spatial filter 31, an imaging lens32, and an imaging element 33.

[0192] The light beam emitted from the light source is obliquelyincident on the surface 1 a as a divergent light beam and illuminatesthe surface 1 a of a silicon wafer OR. The light beam is diffuselyreflected by an impressed pattern on the surface 1 a and specularlyreflected at other portions. The reflected light beam passes through theobjective lens 23 as a converging light beam directed toward thedetection unit 30. The converging light beam passes through the spatialfilter 31 and an imaging lens 32 and an image of the pattern on thesurface 1 a is formed on the imaging element 33 by the diffuselyreflected component. That is, the spatial filter 31 shades thespecularly reflected component.

[0193]FIG. 32 shows a modification of the tenth embodiment in which theprinciple of the arrangement of FIG. 30(B) is applied to an opticalsystem for reading a light-reflection-type object. In particular, theoptical system shown in FIG. 32 does not include the imaging lens 32which is included in the optical system in FIG. 31. In this case, anobjective lens 23 is designed to form the image of the pattern on animaging element 33. Otherwise, the arrangement of the elements in thismodification is the same as the arrangement of the optical system inFIG. 31.

[0194] For this modification, an illumination light beam reaches thesurface 1 a of the silicon wafer OR as a divergent light beam, isreflected at the surface 1 a, and is incident on the objective lens 23.The objective lens 23 converges the reflected light beam and images thepattern on the surface 1 a of the imaging element 33. The spatial filter31 is disposed nearer to the objective lens 23 than an image of thelight source formed by the objective lens 23 and shades the specularlyreflected component of the reflected light beam. Therefore, the image ofthe pattern is formed on the imaging element 33 by the scatteringlyreflected component of the reflected light beam.

Eleventh Embodiment

[0195]FIG. 33 shows an optical system of a pattern reading apparatusaccording to an eleventh embodiment. The optical system includes anillumination unit 10, an objective lens 23, and a detection unit 30. Theillumination unit 10 and the detection unit 30 are disposedsymmetrically with respect to a normal of a surface 1 a so that when alight beam passing through the center of a pinhole which coincides withthe optical axis Ax1 of the illumination unit 10 is specularlyreflected, the light beam coincides with the optical axis Ax2 of thedetection unit 30.

[0196] The illumination unit 10 includes a lamp 11, a pinhole plate 12with a pinhole 12 a to form a minute-area light source, and a diffusionplate 13. The illumination unit 10 is disposed such that an illuminationlight beam is obliquely incident on an object surface at a predeterminedincident angle. The detection unit 30 includes a spatial filter 31, animaging lens 32, and an imaging element 33. The spatial filter 31includes a shading region at the center thereof and is disposed nearerto the objective lens than a paraxial imaging point of the minute-arealight source.

[0197] An illumination light beam emitted from the illumination unit 10is incident on the surface 1 a obliquely as a divergent light beam. Atthe surface 1 a, the illumination light beam is diffusely reflected atan impressed pattern on the surface 1 a and specularly reflected atportions other than the pattern. The reflected light beam passes throughthe objective lens 23 and exits as a converging light beam directedtoward the detection unit 30. At the spatial filter 31, the scatteredreflected component passes through but the specularly reflectedcomponent does not. The scatterered reflected component passes throughthe imaging lens 32 to form an image of the pattern on the surface 1 aon the imaging element 33.

[0198] In this embodiment, the principal plane 32 a of the imaging lens32, the surface 1 a, and the imaging surface 33 a of the imaging element33 are disposed such that imaginary lines extending therefrom cross eachother at an axis RL, as shown by the dashed lines in FIG. 33, based onScheimpflug's rule. Such a disposition eliminates the effect of tilt ofthe image plane, which is conjugate to the surface 1 a, with respect tothe image surface 33 a. As a result, even if a pattern has a width in adirection parallel to a plane including both optical axes Ax1, Ax2, thepattern can be brought into focus as a whole.

[0199] The imaging element 33 converts the image of the pattern into anelectric signal and inputs the signal to an image processing apparatus(not shown). The image processing apparatus displays the image of thepattern on a display screen or analyzes the content of the pattern usinga character recognition algorithm or the like.

[0200] Note that, when the surface 1 a is not parallel with the imagesurface 33 a, as in this embodiment, since a magnification changesdepending upon position in a direction parallel to the plane includingboth the optical axes, a formed pattern is distorted. If the distortionof the image of the pattern affects reading, the distortion can becompensated for by image processing such as an affine transformation orthe like.

[0201]FIG. 34 shows a modification of the optical system of the eleventhembodiment. The optical system in FIG. 34 does not include the imaginglens 32 of FIG. 33 and an objective lens 23 is designed to form an imageof a pattern directly on an imaging element 33. In particular, in thiscase, the principal plane 23 a of the objective lens 23, a surface 1 a,and the image surface 33 a of the imaging element 33 are disposed suchthat imaginary lines extending therefrom cross each other on an axis RL,as shown by the dashed lines in FIG. 34, based on Scheimpflug's rule.Otherwise, the arrangement is the same as that of the optical system ofFIG. 33.

[0202] In this case, the illumination light beam emitted from theillumination unit 10 is incident on the surface 1 a of the silicon waferOR as a divergent light beam and is reflected at the surface 1 a. Theobjective lens 23 converges the reflected light beam and images thepattern on the surface 33 a of the imaging element 33. The spatialfilter 31 is disposed nearer to the objective lens 23 than the paraxialimaging point of the minute-area light source and shades the specularlyreflected component. Thus, the diffusely reflected light beam passesthrough the spatial filter 31 and forms the image of the pattern on theimaging element 33.

[0203] According to the eleventh embodiment, an object surface can bemade conjugate with an imaging surface by disposing the lens having theimaging function and the imaging surface according to Scheimpflug'srule, such that an in-focus pattern image can be obtained.

Twelfth Embodiment

[0204]FIG. 35 shows an optical system for a pattern reading apparatusaccording to a twelfth embodiment. The optical system includes a lightemitting diode 10 a, an objective lens 20, an imaging lens 32, and animaging element 33. The light emitting diode 10 a (light source) isdisposed at a position which is conjugate with a center of curvature ofa surface 1 a of a silicon wafer OR through the objective lens 20. Theimaging lens 32 is disposed at a position which is farther from thesilicon wafer OR than the light emitting diode 10 a. Further, opticalaxes Ax of the objective lens 20 and the imaging lens 32 are coincidentand perpendicular to the surface 1 a. Because the surface 1 a is flat,in the example shown in FIG. 35, the light emitting diode 10 a ispositioned approximately at a focal point of the objective lens 20.

[0205] In the twelfth embodiment, the light beam emitted from the lightemitting diode 10 a passes through the objective lens 20 and illuminatesthe surface 1 a of the silicon wafer OR as a parallel light beamperpendicular to the surface 1 a. The illumination light beam isdiffusely reflected at an impressed pattern on the surface 1 a andspecularly reflected at other portions. The specularly reflectedcomponent is converged to the position of the light emitting diode 10 aas the reflected light beam passes through the objective lens 20 and isshaded by the light emitting diode 10 a.

[0206] The diffusely reflected component is not shaded by the lightemitting diode 10 a and passes through the imaging lens 32 to form animage of the pattern on the imaging element 33.

[0207]FIG. 36 shows a modification of the optical system according tothe twelfth embodiment. As shown in FIG. 36 a light source includes alight emitting element, such as a semiconductor laser 11 a, a lightguide fiber 14, and a coupling lens 15. The semiconductor laser 11 a andthe coupling lens 15 are disposed outside of the optical path betweenthe surface 1 a and the imaging element 33. The light guide fiber 14extends from an entrance end 14 a near the coupling lens 15 to an exitend 14 b disposed at a position which is conjugate with the center ofcurvature of the surface 1 a through the objective lens 20. Since, inFIG. 36, the surface 1 a is flat, the exit end 14 b of the light guidefiber 14 is disposed approximately at the focal point of the objectivelens 20. Otherwise, the arrangement of the optical system is the same asthat of FIG. 35.

[0208] With this modification, the laser beam emitted from thesemiconductor laser 11 a is incident on the entrance end 14 a of thelight guide fiber 14 through the coupling lens 15. Then, the laser beamemitted from the exit end 14 b of the light guide fiber 14 illuminatesthe surface 1 a through the objective lens 20 as a parallel light beam.Since the reflected light beam from the surface 1 a is converged when itpasses through the objective lens 20 again, a specularly reflectedcomponent is shaded by the end of the fiber and only a diffuselyreflected component passes through the imaging lens 32 to form an imageof a pattern on the surface 1 a on the imaging element 33.

[0209] According to the twelfth embodiment, the size of the opticalsystem can be reduced as compared with an optical system in which thelight beam is obliquely incident. Further, the quantity of light islarger than when a beam splitter is used. Still further, the lightsource acts as a spatial filter for shading the specularly reflectedcomponent of light from the object surface so that a distinct image ofthe pattern can be formed by a diffusely reflected component without theprovision of an additional filter.

[0210] The present disclosure relates to subject matter contained inJapanese Patent Applications No. HEI 08-241112, filed on Aug. 23, 1996,No. HEI 08-301076, filed on Oct. 25, 1996, No. HEI 08-342775, filed onDec. 6, 1996, No. HEI 08-342776, filed on Dec. 6, 1996, No. HEI08-342777, filed on Dec. 6, 1996, No. HEI 08-342778, filed on Dec. 6,1996, No. HEI 09-65333, filed on Mar. 4, 1997, No. HEI 09-74497, filedon Mar. 11, 1997, No. HEI 09-65334, filed on Mar. 4, 1997, No. HEI09-134312, filed on May 8, 1997, and No. HEI 09-165422, filed on Jun. 6,1997, which are expressly incorporated herein by reference in theirentirety.

What is claimed is:
 1. A pattern reading apparatus, comprising: anobjective lens disposed opposite an object surface, said object surfacecomprising a reflection surface having a pattern formed thereon as anobject to be read; a minute-area light source positioned to be conjugatewith a center of curvature of the object surface through said objectivelens, for illuminating the object surface through said objective lens;an imaging lens positioned farther from the object surface than saidlight source, with the optical axis of said imaging lens coincident withsaid objective lens; and an imaging element that reads the image of thepattern which is reflected at the object surface and formed through saidobjective lens and said imaging lens.
 2. The pattern reading apparatusaccording to claim 1 , wherein the object surface is approximately flatand said light source is disposed approximately coincident with a focalposition of said objective lens.
 3. The pattern reading apparatusaccording to claim 1 , wherein said light source is a light emittingelement positioned to be conjugate with the center of curvature of theobject surface.
 4. The pattern reading apparatus according to claim 1 ,wherein said light source includes a light emitting element disposedoutside of the optical path between the object surface and said imagingelement and a light guide fiber with the light emitting end thereofpositioned to be conjugate with the center of curvature of the objectsurface for introducing a laser beam from said light emitting elementinto the object surface.